Linear sensor with dual spin valve element having reference layers with magnetization directions different from an external magnetic field direction

ABSTRACT

In one aspect, a linear sensor includes at least one magnetoresistance element that includes a first spin valve and a second spin valve positioned on the first spin valve. The first spin valve includes a first set of reference layers having a magnetization direction in a first direction and a first set of free layers having a magnetization direction in a second direction orthogonal to the first direction. The second spin valve includes a second set of reference layers having a magnetization direction in the first direction and a second set of free layers having a magnetization direction in a third direction orthogonal to the first direction and antiparallel to the second direction. The first direction is neither parallel nor antiparallel to a direction of an expected magnetic field.

BACKGROUND

Magnetic field sensors are used in a variety of applications, including,but not limited to, an angle sensor that senses an angle of a directionof a magnetic field; a current sensor that senses a magnetic fieldgenerated by a current carried by a current-carrying conductor; amagnetic switch that senses the proximity of a ferromagnetic object; arotation detector that senses passing ferromagnetic articles, forexample, magnetic domains of a ring magnet or a ferromagnetic target(e.g., gear teeth) where the magnetic field sensor is used incombination with a back-biased or other magnet; a magnetic field sensorthat senses a magnetic field density of a magnetic field, a linearsensor that senses a position of a ferromagnetic target; and so forth.

In certain applications, magnetic field sensors includemagnetoresistance elements. These elements have an electrical resistancethat changes in the presence of an external magnetic field. Spin valvesare a type of magnetoresistance element formed from two or more magneticmaterials or layers. The simplest form of a spin valve has a reference(or magnetically fixed) layer and a free layer. The resistance of thespin valve changes as a function of the magnetic alignment of thereference and free layers. Typically, the magnetic alignment of thereference layer does not change, while the magnetic alignment of thefree layer moves in response to external magnetic fields.

In some cases, a spin valve may also have a bias. The bias may begenerated by one or more magnetic layers (bias layers) that aremagnetically coupled to the free layer. In the absence of an externalmagnetic field, the bias layers may cause the magnetic alignment of thefree layer to default to a predetermined alignment. The magneticcoupling between the bias layers and the free layer is relatively weakso that an external field can override the bias and realign the magneticalignment of the free layer.

SUMMARY

In one aspect, a linear sensor includes at least one magnetoresistanceelement that includes a first spin valve and a second spin valvepositioned on the first spin valve. The first spin valve includes afirst set of reference layers having a magnetization direction in afirst direction, and a first set of free layers having a magnetizationdirection in a second direction orthogonal to the first direction. Thesecond spin valve includes a second set of reference layers having amagnetization direction in the first direction, and a second set of freelayers having a magnetization direction in a third direction orthogonalto the first direction and antiparallel to the second direction. Thefirst direction is neither parallel nor antiparallel to a direction ofan expected magnetic field. The at least one magnetic resistance elementdetects magnetic field changes caused by a target object enabling thelinear sensor to provide a differential signal indicating a linearposition of the target object.

In another aspect, a linear sensor includes a first bridge and a secondbridge. The first bridge includes a first spin valve, a second spinvalve in series with the first spin valve, a third spin valve inparallel with the first spin valve and a fourth spin valve in serieswith the third spin valve. The second bridge includes a fifth spinvalve, a sixth spin valve in series with the fifth spin valve element, aseventh spin valve in parallel with the fifth spin valve and an eighthspin valve in series with the seventh spin valve. The first spin valveincludes a first set of reference layers having a magnetizationdirection in a first direction, and a first set of free layers having amagnetization direction in a second direction orthogonal to the firstdirection. The fifth spin valve is positioned on the first spin valve.The fifth spin valve includes a second set of reference layers having amagnetization direction in the first direction, and a second set of freelayers having a magnetization direction in a third direction orthogonalto the first direction and antiparallel to the second direction. Thefirst direction is neither parallel nor antiparallel to a direction ofan external magnetic field. The first and second bridges detect magneticfield changes caused by a target object enabling the linear sensor toprovide a differential signal indicating a linear position of the targetobject.

In a further aspect, a method includes forming a linear sensor thatincludes a first bridge that includes a first set of spin valves and asecond bridge that includes a second set of spin valves. The first setof spin valves each includes a first set of reference layers having amagnetization direction in a first direction. The second set of spinvalves each includes a second set of reference layers having amagnetization direction in the first direction. The first direction isneither parallel nor antiparallel to a direction of an expected magneticfield. The first direction and the direction of the expected magneticfield form a tilt angle, wherein the tilt angle is greater than 0°. Themethod further includes measuring a sensitivity of a first bridge tochanges in a magnetic field; measuring a sensitivity of a second bridgeto changes in the magnetic field; and weighting an output of the firstbridge and an output of the second bridge based on the measuredsensitivities of the first and second bridges.

DESCRIPTION OF THE DRAWINGS

The foregoing features may be more fully understood from the followingdescription of the drawings. The drawings aid in explaining andunderstanding the disclosed technology. Since it is often impractical orimpossible to illustrate and describe every possible embodiment, theprovided figures depict one or more illustrative embodiments.Accordingly, the figures are not intended to limit the scope of thebroad concepts, systems and techniques described herein. Like numbers inthe figures denote like elements.

FIG. 1 is a diagram of an example of a dual double-pinned spin valveelement;

FIG. 2 is a diagram of an example of the magnetization directions ofbias and reference layers for both spin valves shown in FIG. 1 withrespect to a magnetic field direction;

FIG. 3A is a circuit diagram of an example of a bridge with spin valvesof a first type;

FIG. 3B is a circuit diagram of an example of a bridge with spin valvesof a second type;

FIG. 4 is a block diagram of an example of a linear sensor that includesbridges of FIGS. 3A and 3B;

FIG. 5 is a flowchart of an example of a process to weight bridgeoutputs;

FIG. 6 is a graph of an example of weighting that may be used to weightbridge outputs;

FIG. 7 is a block diagram of another example of a linear sensor thatweights bridge outputs;

FIG. 8 is a graph of an example of changes in linear range with respectto changes in sensitivity for different tilt angles;

FIG. 9 is a graph of an example of changes in resistance using an erffunction with different tilt angles;

FIG. 10 is a graph of an example of changes in linear range andsensitivity for different erf function widths used for weighting bridgeoutputs; and

FIG. 11 is a block diagram of an example of a weighting processor onwhich the process of FIG. 5 may be implemented.

DETAIL DESCRIPTION

Described herein are techniques to fabricate a linear sensor that has anincreased linear range than traditional linear sensors that use spinvalves without necessarily trading off sensitivity of the linear sensorsto detect changes in a magnetic field. In one particular example, thelinear sensor includes a dual spin valve element that has (1) referencelayers that have magnetization directions that are neither parallel norantiparallel to an external magnetic field, and (2) free layers thathave magnetization directions that are opposite from each other andorthogonal to their respective reference layers. As used herein the term“linear range” refers to a range where the changes in resistivity, forexample, of an MR element are linear with respect to changes in anintensity of a magnetic field.

As used herein, the term “target object” is used to describe an objectto be sensed or detected by a magnetic field sensor or magnetic fieldsensing element. The target object may include a conductive materialthat allows for eddy currents to flow within the target, for example ametallic target that conducts electricity.

An example of a magnetic field sensor is a linear sensor. A linearsensor detects magnetic field changes caused by the target objectenabling the linear sensor to provide a differential signal that varieslinearly with the position of the target object.

Referring to FIG. 1, an MR element 100 is an example of a dualdouble-pinned spin valve element. The MR element 100 is deposited orotherwise provided upon a substrate 101 and includes a plurality oflayers. The plurality of layers includes a nonmagnetic seed layer 102disposed over the substrate 101, a first spin valve 101 a disposed overthe nonmagnetic seed layer 102 and a second spin valve 101 b disposedover the first spin valve 101 a. The plurality of layers also includesan antiferromagnetic pinning layer 120, which is shared by the firstspin valve 101 a and the second spin valve 101 b, and a nonmagnetic caplayer 104 disposed over the second spin valve 101 b.

The first spin valve 101 a includes bias layers 110, free layer 114 andreference layers 116. The bias layers 110 includes an antiferromagneticpinning layer 111 disposed over the nonmagnetic seed layer 102 and aferromagnetic pinned layer 112 disposed over the antiferromagneticpinning layer 111. The first spin valve 101 a also includes anonmagnetic spacer layer 113 disposed over the ferromagnetic pinnedlayer 112 with the free layers 114 structure 114 disposed over thenonmagnetic spacer layer 113. The free layers 114 includes a firstferromagnetic free layer 114 a and a second ferromagnetic free layer 114b disposed over the first ferromagnetic free layer 114 a.

The first spin valve 101 a further includes a nonmagnetic spacer layer115 disposed over the free layers 114 with the reference layers 116disposed over the nonmagnetic spacer layer 115. The reference layers 116includes a ferromagnetic layer 116 a, a ferromagnetic pinned layer 116 cand a nonmagnetic spacer layer 116 b disposed therebetween.

The second spin valve 101 b, which is similar to the first spin valve101 a, but includes layers that are in a substantially reverse order orarrangement as the layers which are shown in the first spin valve 101 awith respect to the seed layer 102, includes reference layers 131disposed over the antiferromagnetic pinning layer 120, a nonmagneticspacer layer 132 disposed over the reference layers 131 and free layers133 disposed over the nonmagnetic spacer layer 132. The reference layers131 includes a first ferromagnetic layer 131 a, a second ferromagneticpinned layer 131 c and a nonmagnetic spacer layer 131 b disposedtherebetween. Additionally, the free layers 133 includes a firstferromagnetic free layer 133 a and a second ferromagnetic free layer 133b disposed over the first ferromagnetic free layer 133 a.

The second spin valve 101 b also includes bias layers 130. The biaslayer 130 includes nonmagnetic spacer layer 134 disposed over the freelayers 133, a ferromagnetic pinned layer 135 disposed over thenonmagnetic spacer layer 134 and an antiferromagnetic pinning layer 136disposed over the ferromagnetic pinned layer 135. A nonmagnetic caplayer 104 is disposed over the antiferromagnetic pinning layer 136.

Each of the layers in MR element 100 includes one or more respectivematerials (e.g., magnetic materials) and has a respective thickness, asshown. Materials of the layers are shown by atomic symbols.Additionally, thicknesses of the layers are shown in nanometers. Inother embodiments, the material and thicknesses of the layers in MRelement 100 may be replaced with other materials and thicknesses.

Referring to FIG. 2, while the stack of FIG. 1 is known in the art (see,for example, FIG. 20 of U.S. Pat. No. 9,812,637), magnetizationdirections of certain layers as described herein are not known in theart. For example, a magnetization direction 204 a of reference layers(e.g., reference layers 116 (FIG. 1)) of a first spin valve (e.g., spinvalve 101 a (FIG. 1)) and a magnetization direction 204 b of referencelayers (e.g., reference layers 131 (FIG. 1)) of a second spin valve(e.g., spin valve 101 b (FIG. 1)) are each tilted with respect to amagnetic field direction 202 forming a tilt angle that is neitherparallel nor antiparallel with respect to the magnetic field direction202.

Also, a magnetization direction 206 a of free layers (e.g., free layers114 (FIG. 1)) of the first spin valve (e.g., spin valve 101 a (FIG. 1))is (1) antiparallel to a magnetization direction 206 b of free layers(e.g., free layers 133 (FIG. 1)) of the second spin valve (e.g., spinvalve 101 b (FIG. 1)); and (2) orthogonal to the magnetization direction204 a of the first spin valve. Likewise, the magnetization direction 206b of free layers of the second spin valve is (1) antiparallel to themagnetization direction 206 a of the first spin valve; and (2)orthogonal to the magnetization direction 204 b of the second spinvalve. In some examples, the magnetization directions 204 a, 204 b ofthe reference layers are offset from the magnetic field direction 202 bybetween 10° to 50°.

The linear range of a dual spin valve element increases by tilting themagnetization directions 204 a, 204 b with respect to the magnetic field202, and having the magnetization directions 206 a, 206 b beingantiparallel to each other and orthogonal to the magnetizationdirections 204 a, 204 b. The increased linear range does not reduce thesensitivity of the dual spin valve element to detecting changes in amagnetic field. As further described herein, these dual spin valveelements may be used in bridges in a linear sensor to increase thelinear range of the linear sensor without reducing the sensitivity ofthe linear sensor to detect changes in a magnetic field.

Referring to FIG. 3A, a bridge 302 a includes a first spin valve 304 a,a second spin valve 306 a in series with the first spin valve 304 a, athird spin valve 308 a in parallel with the first spin valve 304 a, anda fourth spin valve 310 a in series with the third spin valve 308 a. Thebridge 302 a also includes a first output terminal 350 a and a secondoutput terminal 350 b that provide an output signal. The bridge 302 amay be a voltage bridge, a current bridge, or a conductance bridge.

Each of the spin valves 304 a, 306 a, 308 a, 310 a are a first type.That is, the spin valves 304 a, 306 a, 308 a, 310 a are fabricated to bethe same. In one particular example, the spin valves 304 a, 306 a, 308a, 310 a are fabricated to be the same as the spin valve 101 a (FIG. 1).Also, each of spin valves 304 a, 306 a, 308 a, 310 a have the samemagnetization direction 314 a of the reference layers and the samemagnetization direction 316 a of the free layers.

Referring to FIG. 3B, a bridge 302 b includes a first spin valve 304 b,a second spin valve 306 b in series with the first spin valve 304 b, athird spin valve 308 b in parallel with the first spin valve 304 b, anda fourth spin valve 310 b in series with the third spin valve 308 b. Thebridge 302 b also includes a first output terminal 360 a and a secondoutput terminal 360 b that provide an output signal. The bridge 302 bmay be a voltage bridge, a current bridge, or a conductance bridge.

Each of the spin valves 304 b, 306 b, 308 b, 310 b are a second type.That is, the spin valves 304 b, 306 b, 308 b, 310 b are fabricated to bethe same. In one particular example, the spin valves 304 b, 306 b, 308b, 310 b are the same as the spin valve 101 b (FIG. 1). Also, each ofspin valves 304 b, 306 b, 308 b, 310 b have the same magnetizationdirection 314 b of the reference layers and the same magnetizationdirection 316 b of the free layers.

While the description of the bridges 302 a, 302 b were given using thespin valves of FIG. 1 as an example, other spin valves may be used otherthan those described in FIG. 1. Also, the spin valves of a first typeand a second type need not be fabricated one on top of another. Rather,the spin valves of each type may be constructed side-by-side, forexample, or spaced apart by some distance.

The spin valves of a first type may be either a giant magnetoresistanceelement (GMR) or a tunneling magnetoresistance element (TMR). The spinvalves of a second type may be either a GMR or a TMR.

Referring to FIG. 4, an example of a linear sensor is a linear sensor402. The linear sensor 402 includes the bridges 302 a, 302 b andprocessing circuitry 420. The bridges 302 a, 302 b each detects magneticfield changes caused by a target object 404 interacting with themagnetic field 202. The processing circuitry 420 processes the outputsignals from each bridge 302 a, 302 b to provide a signal 440, which isa differential signal indicating a linear position of the target object.

Referring to FIG. 5, in other embodiments, it may be desirable to weightthe outputs of the bridges 302 a, 302 b. For example, if one bridge ismore sensitive to changes in a magnetic field than another bridge, thebridge that is more sensitive is weighted higher than the bridge that isnot as sensitive. An example, of a process to weight output of bridges(e.g., bridges 302 a, 302 b (FIGS. 3A and 3B) is a process 500.

Process 500 measures sensitivity of a first bridge (502). For example,the output signal at the terminals 350 a, 350 b of the bridge 302 a(FIG. 3A) is measured.

Process 500 measures sensitivity of a second bridge (506). For example,the output signal at the terminals 360 a, 360 b of the bridge 302 b(FIG. 3B) is measured.

Process 500 weights the outputs of the first and second bridges based onthe measured sensitivities (510). For example, the outputs of thebridges 302 a, 302 b are compared and the bridge with the larger signaloutput receives the higher weight.

Referring to FIG. 6, a graph 600 may be used for weighting in processingblock 510 (FIG. 5). The graph 600 includes a weighting curve 604corresponding to the bridge 302 a (FIG. 3A) and a weighting curve 610corresponding to the bridge 302 b (FIG. 3B). In one particular example,if the output of the bridge 302 a is −10, then the output of the bridge302 a is weighted 0.25 and the output of the second bridge is weighted0.75.

Referring to FIG. 7, another example of a linear sensor is a linearsensor 402′. The linear sensor 402′ includes the bridges 410 a, 410 band processing circuitry 420′. The bridges 410 a, 410 b each detectsmagnetic field changes caused by the target object 404 interacting withthe magnetic field 202. The processing circuitry 420′ includes weightingcircuitry 710. The processing circuitry 420′processes the output signalsfrom each bridge 410 a, 410 b and weights the output signals from thebridges 410 a, 410 b using the weighting circuitry 710 to provide asignal 440′, which is a differential signal indicating a linear positionof the target object.

Referring to FIG. 8, a graph 800 depicts changes of sensitivity So withrespect to changes in a linear range for different tilt angles. Thecurve 802 is a plot taken at different tilt angles. The point 812corresponds to a 30° tilt angle and the point 816 corresponds to a 0°tilt angle. Points between the point 802 and the point 808 are linearlyspaced between 0° and 30° by 2° steps.

Referring to FIG. 9, a graph 900 is an example that depicts howresistance changes using an erf function weight scheme with various tiltangles of the spin valves (e.g., spin valves 304 a, 306 a, 308 a, 310 a(FIG. 3A) and spin valves 304 b, 306 b, 308 b, 310 b (FIG. 3B)) used ina bridge (e.g., bridge 302 a (FIG. 2A) and bridge 302 b (FIG. 2B)). FIG.9 may be used in weighting the outputs of the bridges (e.g., bridges 410a, 410 b (FIG. 7)). The graph 900 is an example of depicting how thelinear range may be increased while the sensitivity may be keptrelatively constant. In this example, the width of the erf function is45 Oe.

Referring to FIG. 10, a graph 1000 depicts changes in linear range andsensitivity for different erf function widths. The Graph 1000 is used inweighting bridge outputs (e.g., the outputs of the bridges 410 a, 410 b(FIG. 7)).

A curve 1002 uses a weighting scheme with an erf function with a widthof zero, which is equivalent to a Heaviside function. A curve 1006 usesa weighting scheme with an erf function with a width of 40 Oe. A curve1010 uses a weighting scheme with an erf function with a width of 70 Oe.

Each measured point in the curves 1002, 1006, 1010 represents adifferent tilt angle. A point 1020 shared by the curves 1002, 1006, 1010is a zero-degree tilt angle. For each curve 1002, 1006, 1010 moving leftto right in the graph 1000, each new point is an increase of 2 degrees.For example, in the curve 1002, a point 1024 is at a 2° tilt angle, apoint 1026 is at a 4° tilt angle, a point 1028 is at an 6° tilt angle, apoint 1030 is at an 8° tilt angle, a point 1032 is at a 10° tilt angle,a point 1034 is at a 12° tilt angle, and a point 1036 is at a 14° tiltangle. In another example, in the curve 1006, a point 1040 is at a 2°tilt angle, a point 1042 is at a 4° tilt angle, and so forth to thepoint 1044 with a 16° tilt angle. In a further example, in the curve1010, a point 1050 is at a 2° tilt angle, a point 1052 is at a 4° tiltangle, and so forth to the point 1054 with a 24° tilt angle.

The curve 1002 is the most sensitive of the three curves, 1002, 1006,1010 in graph 1000. The curve 1006 provides negligible loss ofsensitivity while maximizing the linear range by 54%. The curve 1010provides the highest linear range (+68%) of the three curves, 1002,1006, 1010 while losing a little sensitivity.

The graph 1000 is measured on a bridge with spin valves where each spinvalve has a stack bias of 84 Oersted (Oe). The stack bias is theamplitude of the bias field applied to the free layer of a spin valve.In general, the higher the bias field, the large the linear range andthe smaller the sensitivity to a magnetic field. Thus, the width of theerf function is scaled according to the stack bias in order to achievethe same result. For example, for a stack bias that is 168 Oe, the erffunction width is doubled.

In traditional schemes, if one multiplies by X the linear range, thenone would multiply by 1/X to get the new sensitivity. For example, toobtain 70% more linear range, the sensitivity would be decreased by 42%using traditional techniques. However, using the techniques describedherein, the sensitivity would only be reduced by 4% (see, for example,curve 1010). In another example, to get 38% more linear range usingtraditional techniques, 28% of the sensitivity would be lost. However,using the techniques described herein, the sensitivity instead would beincreased by only 2.8% (see, for example, curve 1002).

Referring to FIG. 11, an example of weighting circuitry 710 (FIG. 7) isweighting processor 1100, which includes a processor 1102, a volatilememory 1104, a non-volatile memory 1106 (e.g., hard disk) and the userinterface (UI) 1108 (e.g., a graphical user interface, a mouse, akeyboard, a display, touch screen and so forth). The non-volatile memory1106 stores computer instructions 1112, an operating system 1116 anddata 1118. In one example, the computer instructions 1112 are executedby the processor 1102 out of volatile memory 1104 to perform all or partof the processes described herein (e.g., process 500).

The processes described herein (e.g., process 500) are not limited touse with the hardware and software of FIG. 11; they may findapplicability in any computing or processing environment and with anytype of machine or set of machines that can run a computer program. Theprocesses described herein may be implemented in hardware, software, ora combination of the two. The processes described herein may beimplemented in computer programs executed on programmablecomputers/machines that each includes a processor, a non-transitorymachine-readable medium or other article of manufacture that is readableby the processor (including volatile and non-volatile memory and/orstorage elements), at least one input device, and one or more outputdevices. Program code may be applied to data entered using an inputdevice to perform any of the processes described herein and to generateoutput information.

The system may be implemented, at least in part, via a computer programproduct, (e.g., in a non-transitory machine-readable storage medium),for execution by, or to control the operation of, data processingapparatus (e.g., a programmable processor, a computer, or multiplecomputers)). Each such program may be implemented in a high-levelprocedural or object-oriented programming language to communicate with acomputer system. However, the programs may be implemented in assembly ormachine language. The language may be a compiled or an interpretedlanguage and it may be deployed in any form, including as a stand-aloneprogram or as a module, component, subroutine, or other unit suitablefor use in a computing environment. A computer program may be deployedto be executed on one computer or on multiple computers at one site ordistributed across multiple sites and interconnected by a communicationnetwork. A computer program may be stored on a non-transitorymachine-readable medium that is readable by a general or special purposeprogrammable computer for configuring and operating the computer whenthe non-transitory machine-readable medium is read by the computer toperform the processes described herein. For example, the processesdescribed herein may also be implemented as a non-transitorymachine-readable storage medium, configured with a computer program,where upon execution, instructions in the computer program cause thecomputer to operate in accordance with the processes. A non-transitorymachine-readable medium may include but is not limited to a hard drive,compact disc, flash memory, non-volatile memory, volatile memory,magnetic diskette and so forth but does not include a transitory signalper se.

The processes described herein are not limited to the specific examplesdescribed. For example, the process 500 is not limited to the specificprocessing order of FIG. 5, respectively. Rather, any of the processingblocks of FIG. 5 may be re-ordered, combined, or removed, performed inparallel or in serial, as necessary, to achieve the results set forthabove.

The processing blocks (for example, the process 500) associated withimplementing the system may be performed by one or more programmableprocessors executing one or more computer programs to perform thefunctions of the system. All or part of the system may be implementedas, special purpose logic circuitry (e.g., an FPGA (field-programmablegate array) and/or an ASIC (application-specific integrated circuit)).All or part of the system may be implemented using electronic hardwarecircuitry that include electronic devices such as, for example, at leastone of a processor, a memory, programmable logic devices or logic gates.

Elements of different embodiments described herein may be combined toform other embodiments not specifically set forth above. Variouselements, which are described in the context of a single embodiment, mayalso be provided separately or in any suitable subcombination. Otherembodiments not specifically described herein are also within the scopeof the following claims.

What is claimed is:
 1. A linear sensor comprising: at least onemagnetoresistance element comprising: a first spin valve comprising: afirst set of reference layers having a magnetization direction in afirst direction; and a first set of free layers having a magnetizationdirection in a second direction orthogonal to the first direction; and asecond spin valve positioned on the first spin valve, the second spinvalve comprising: a second set of reference layers having amagnetization direction in the first direction; and a second set of freelayers having a magnetization direction in a third direction orthogonalto the first direction and antiparallel to the second direction, whereinthe first direction is neither parallel nor antiparallel to a directionof an expected magnetic field, and wherein the at least one magneticresistance element detects magnetic field changes caused by a targetobject enabling the linear sensor to provide a differential signalindicating a linear position of the target object.
 2. The linear sensorof claim 1, wherein the first set of reference layers and the second setof reference layers share one or more layers.
 3. The linear sensor ofclaim 2, wherein the first spin valve further comprises a first set ofbias layers, wherein the first set of free layers is positioned betweenthe first set of reference layers and the first set of bias layers, andwherein the second spin valve further comprises a second set of biaslayers, wherein the second set of free layers is positioned between thesecond set of reference layers and the second set of bias layers.
 4. Thelinear sensor of claim 1, wherein the first spin valve is either a giantmagnetoresistance element (GMR) or a tunneling magnetoresistance element(TMR).
 5. The linear sensor of claim 4, wherein the second spin valve iseither a giant magnetoresistance element (GMR) or a tunnelingmagnetoresistance element (TMR).
 6. The linear sensor of claim 1,wherein the first direction and the direction of the expected magneticfield form a tilt angle, wherein the tilt angle is between 0° and 30°.7. The linear sensor of claim 6, wherein the tilt angle enables thelinear sensor to have a linear range more than 65% greater than a linearsensor with a tilt angle of 0° and to have a sensitivity no more than 4%less than a sensitivity of the linear sensor with the tilt angle of 0°.8. The linear sensor of claim 6, wherein the tilt angle enables thelinear sensor to have a linear range more than 50% greater than a linearsensor with a tilt angle of 0° and to have a sensitivity no more than0.25% less than a sensitivity of the linear sensor with the tilt angleof 0°.
 9. The linear sensor of claim 6, wherein the tilt angle enablesthe linear sensor to have a linear range more than 35% greater than alinear sensor with a tilt angle of 0° and to have a sensitivity morethan 2.5% greater than a sensitivity of the linear sensor with the tiltangle of 0°.
 10. A linear sensor comprising: a first bridge comprising:a first spin valve; a second spin valve in series with the first spinvalve; a third spin valve in parallel with the first spin valve; and afourth spin valve in series with the third spin valve; and a secondbridge comprising: a fifth spin valve; a sixth spin valve in series withthe fifth spin valve element; a seventh spin valve in parallel with thefifth spin valve; and an eighth spin valve in series with the seventhspin valve, wherein the first spin valve comprises: a first set ofreference layers having a magnetization direction in a first direction;a first set of free layers having a magnetization direction in a seconddirection orthogonal to the first direction; and wherein the fifth spinvalve is positioned on the first spin valve, wherein the fifth spinvalve comprises: a second set of reference layers having a magnetizationdirection in the first direction; a second set of free layers having amagnetization direction in a third direction orthogonal to the firstdirection and antiparallel to the second direction, wherein the firstdirection is neither parallel nor antiparallel to a direction of anexternal magnetic field, wherein the first and second bridges detectmagnetic field changes caused by a target object enabling the linearsensor to provide a differential signal indicating a linear position ofthe target object.
 11. The linear sensor of claim 10, wherein the first,second, third and fourth spin valves are electrically the same.
 12. Thelinear sensor of claim 11, wherein the fifth, sixth, seventh and eighthspin valves are electrically the same.
 13. The linear sensor of claim12, wherein the sixth spin valve is positioned on the second spin valve,wherein the seventh spin valve is positioned on the third spin valve,and wherein the eighth spin valve is positioned on the fourth spinvalve.
 14. The linear sensor of claim 10, wherein the first set ofreference layers and the second set of reference layers share one ormore layers.
 15. The linear sensor of claim 14, wherein the first spinvalve further comprises a first set of free layers positioned betweenthe first set of reference layers and the first set of bias layers, andwherein the second spin valve further comprises a second set of freelayers positioned between the second set of reference layers and thesecond set of bias layers.
 16. The linear sensor of claim 10, whereinthe first spin valve is either a giant magnetoresistance element (GMR)or a tunneling magnetoresistance element (TMR).
 17. The linear sensor ofclaim 16, wherein the second spin valve is either a giantmagnetoresistance element (GMR) or a tunneling magnetoresistance element(TMR).
 18. The linear sensor of claim 10, wherein the output of thefirst bridge and the second bridge are weighted with respect to eachother based on their respective sensitivities.
 19. The linear sensor ofclaim 10, wherein the first bridge is a voltage bridge, and the secondbridge is a voltage bridge.
 20. The linear sensor of claim 10, whereinthe first bridge is a current bridge, and the second bridge is a currentbridge.
 21. The linear sensor of claim 10, wherein the first bridge is aconductance bridge, and the second bridge is a conductance bridge. 22.The linear sensor of claim 10, wherein the first direction and thedirection of the expected magnetic field form a tilt angle, wherein thetilt angle is between 0° and 30°.
 23. The linear sensor of claim 22,wherein the tilt angle enables the linear sensor to have a linear rangemore than 65% greater than a linear sensor with a tilt angle of 0° andto have a sensitivity no more than 4% less than a sensitivity of thelinear sensor with the tilt angle of 0°.
 24. The linear sensor of claim22, wherein the tilt angle enables the linear sensor to have a linearrange more than 50% greater than a linear sensor with a tilt angle of 0°and to have a sensitivity no more than 0.25% less than a sensitivity ofthe linear sensor with the tilt angle of 0°.
 25. The linear sensor ofclaim 22, wherein the tilt angle enables the linear sensor to have alinear range more than 35% greater than a linear sensor with a tiltangle of 0° and to have a sensitivity more than 2.5% greater than asensitivity of the linear sensor with the tilt angle of 0°.
 26. A methodcomprising, forming a linear sensor comprising: a first bridgecomprising a first set of spin valves each comprising a first set ofreference layers having a magnetization direction in a first direction;and a second bridge comprising a second set of spin valves eachcomprising a second set of reference layers having a magnetizationdirection in the first direction, wherein the first direction is neitherparallel nor antiparallel to a direction of an expected magnetic field,wherein the first direction and the direction of the expected magneticfield form a tilt angle, wherein the tilt angle is greater than 0°;measuring a sensitivity of a first bridge to changes in a magneticfield; measuring a sensitivity of a second bridge to changes in themagnetic field; and weighting an output of the first bridge and anoutput of the second bridge based on the measured sensitivities of thefirst and second bridges.
 27. The method of claim 26, wherein the tiltangle enables the linear sensor to have a linear range more than 65%greater than a linear sensor with a tilt angle of 0° and to have asensitivity no more than 4% less than a sensitivity of the linear sensorwith the tilt angle of 0°.
 28. The method of claim 26, wherein the tiltangle enables the linear sensor to have a linear range more than 50%greater than a linear sensor with a tilt angle of 0° and to have asensitivity no more than 0.25% less than a sensitivity of the linearsensor with the tilt angle of 0°.
 29. The method of claim 26, whereinthe tilt angle enables the linear sensor to have a linear range morethan 35% greater than a linear sensor with a tilt angle of 0° and tohave a sensitivity more than 2.5% greater than a sensitivity of thelinear sensor with the tilt angle of 0°.